The present invention relates to an ultraviolet curing resin suitable for use in a protective film of an optical disk such as a compact disk, a magneto-optical disk or the like.
An optical disk such as the compact disk, the magneto-optical disk or the like has a structure including a metal thin film layer formed by or evaporation or sputtering on a transparent substrate made of a material such as polycarbonate or the like. Since the metal thin film layer is easily deteriorated in characteristics because of influence of oxygen and vapor in the air, an ultraviolet curing resin and so on are used as the protective film. A spin coating method is generally employed as a coating method therefor.
As a film thickness of the protective film for a recording layer of the optical disk becomes thick, a substrate of the disk tends to be warped. Accordingly, a reproducing apparatus misreads or is prevented from reading the recorded information. Therefore, the film thickness of the protective film should be set thinner. For this end, a composition to be coated used in the spin coating method should have low viscosity.
However, since particularly a compound having low molecular weight and low viscosity used in a light curing resin material generally has a high primary irritation index (PII) value, there is often reported the problem that a maintenance person for a manufacturing apparatus touches the light curing resin material and then gets a rash.
A protective coating agent for an optical disk used to reduce skin irritations is proposed in Japanese laid-open publication No. 2-6562. The protective coating agent includes a compound made by denaturing an acrylate compound having low molecular weight with an ethylene oxide, such as an adduct of a trimethylolpropane triacrylate with 3-mol ethylene oxide. However, the protective coating agent for the optical disk is encountered by the problem that it has high viscosity because of its high molecular weight.
If the PII value of the compound is set lower, then the compound is lowered in a corrosion resistance and cannot serve as the protective film. Further, the protective film having the low PII value is encountered by the problem that after the compound is cured, it strongly stinks to aggravate the working conditions.
Indeed, it is desirable in view of productivity that the coating agent has low viscosity and low surface tension which improves a wetting property relative to a disk substrate. But, since bubbles are easily produced because of an additive added in order to lower the surface tension of the coating agent, there is then the problem of aggravation of the productivity such as frequent production of substandard disks, low yield and so on. Further, since the additive oozes onto the protective film, there is then the problem that print adhesion is lowered to cause disadvantages such as detachment of a printing ink therefrom or the like.